1. Technical Field
This invention relates to the technology of purifying exhaust gases or emissions of stationary and mobile sources, and more particularly to the technology of cleansing such emissions of NO.sub.x from a stream which may vary from oxygen-deficient (fuel-rich) to oxygen-rich (fuel-lean) or lean burn.
2. Discussion of Related Art
Emissions from large scale stationary sources, containing NO.sub.x and excess O.sub.2, are generally treated with ammonia as a reductant over a catalyst containing V.sub.2 O.sub.5 on Ti0.sub.2 (see H. Bosch and F. Janssen, "Catalysis Today", Vol. 1(4), 1987). Emissions from mobile sources that do not have excess oxygen but contain NO.sub.x (automotive vehicles) are removed by reaction with in-situ reductants, such as carbon monoxide or hydrocarbons (HC), when passed over a catalyst, often containing rhodium. Such a catalyst would be ineffective in the presence of a large excess of O.sub.2 (see K. C. Taylor, "Automobile Catalyst Converters", Springer, Berlin, 1984).
Recently, copper-exchanged zeolites have been used to reduce NO.sub.x in the presence of excess oxygen (see U.S. Pat No. 4,934,142 and Japanese patent application publication No. Hei 3-52644, 3/6/91), but to attain substantial conversion efficiencies at the moderate temperatures of an exhaust produced by a lean-burn engine, a temporary fuel rich condition is required to provide a residual HC reductant. Unfortunately, it is not desirable to operate an automotive engine or other emission source under artificially fuel rich conditions simply to facilitate catalytic conversion of the engine emissions (see M. Iwamoto et al, Applied Catalysis, Vol. 69. L 15-19, 1991). To facilitate conversion of NO.sub.x under conditions of excess oxygen, without the need for stimulating high HC in the exhaust, alkane and alkene additions have been suggested as reductants injectable into the emissions ahead of the zeolite (see S. Sato et al. Applied Catalysis, Vol. 70, L 1-5 (1991)). However, it is difficult to meter small doses of such gaseous reductants to match accurately the varying NO.sub.x content of the exhaust gas and to reliably and safely store such gaseous reductants on-board a vehicle.
In an effort to reduce NO.sub.x emissions, Montreuil et al., in U.S. Pat 5,149,511 discloses injecting a water-soluble, oxygenated hydrocarbon such as propanol as a reductant into exhaust gas emissions having excess oxygen; the emissions then are exposed to a transition metal-exchanged high silica ZSM5 zeolite.
It would be advantageous if a reductant could be employed which doesn't require oxygen in the fuel stream and yet converts a high percentage of NO.sub.x. It would be further advantageous if such a reductant would also be effective in converting NO.sub.x in a fuel-lean (oxygen rich) environment. That is, such a reductant would extend the conversion range for NO.sub.x from oxidizing (fuel lean) to reducing (fuel rich) conditions. It would further be desirable to provide a reductant which has little affect on reducing the activity of the catalyst during operation. The present invention catalyst provides these advantages.